Polyaxial screw assemblies are often used in spine fixation to stabilize the lumbar spine and promote bony fusion. Polyaxial screw assemblies can also be used as a possible anchor point for a dynamic system. In both approaches, the polyaxial screw assembly is implanted by establishing access through a posterior approach to the thoraco-lumbar spine. Many posterior procedures are done with an open surgical method, meaning that the skin of the patent is incised from the cranial aspect of the area to be treated to the caudal aspect. This can require a significantly long incision, potentially resulting in trauma to the muscles, nerves and other soft tissue of the back. This trauma can lead to biomechanical instability, greater possible necrosis, and an increased time for recovery.
Minimally invasive surgery (MIS) attempts to minimize the damage that the insertion of these implants causes through the use of smaller incisions and muscle splitting rather than cutting. The smallest footprint of the MIS family is referred to as percutaneous surgery, characterized by stab incisions for the introduction of the screw into the patient. MIS surgery in general, and percutaneous surgery in particular, make use of instruments called “downtubes”, which can be looked at as temporary extensions of the screw body that communicate from the surgical site through to the surface of the skin. These tubes are removed once the surgery is complete.
Despite their advantages, conventional downtubes have a number of drawbacks. Many conventional downtubes fail to securely engage the screw body and remain in place during a procedure. In addition, conventional downtubes often feature a number of movable or sliding parts that are interconnected. Movable or sliding parts can make operation more complicated, and can be prone to binding and jamming with other parts.
Downtubes with multiple parts also create burdens prior to surgery, because the parts must be disassembled so that they can be cleaned and sterilized thoroughly prior to being used. Multiple parts also tend to increase the overall footprint size of the downtube, which is undesirable in minimally invasive procedures. Moreover, downtubes become more costly to manufacture as the number of parts increases. More parts generally require more manufacturing steps, increasing the probability of manufacturing error. In addition, parts can be lost during reprocessing of devices made from multiple components.
Given the drawbacks of known downtubes, there is a need for an improved downtube that is easier to use, less prone to complications, and less costly to manufacture.